This invention relates to indicator devices for indicating the time integral of a monitored parameter.
One particular application of the device is for indicating the time temperature integral of degradable or perishable commodities. Such devices may be placed on wrappings or containers of perishable commodities to give a visual indication of the integral of time and temperature to which the commodity has been subjected since packaging, that is, since the fresh condition, in particular for indicating when the commodity has been subject to conditions of time and temperature such that it will have suffered deterioration or spoilage. Such devices are generally known as "time temperature indicators".
In general, the common requirements of a time temperature indicator are that it should exhibit properties which are readily changed by time and/or temperature, give a visual indication of time and temperature integration, and should be activatable when the commodity is fresh or has been recently packaged. Devices which fulfil these requirements are already known and are based on processes such as chemical diffusion, physical diffusion, thermochromic effects, biological reactions, polymer crystallization and electrochemical effects.
Another effect, known as "persistent electrochromism", is known (see U.S. Pat. No. 3,521,941), according to which the electromagnetic radiation absorption properties of a material are altered when subjected to an electric field or injection of charge, thereby causing a colour change in the material. An electrochromic device requiring no externally applied electric power source is disclosed in U.S. Pat. No. 3,940,205, according to which a thin film of tungsten trioxide is deposited on a glass slide and an indium electrode contacts the tungsten trioxide electrochromic layer with an acid electrolyte in contact with the electrochromic layer and the electrode. An internal electrochemical cell is set up so as to produce colouration of the electrochromic material.